Adding support for lowpass filter

[jesusdpa1]

Adding lowpass filter class, useful for processing emg recordings.

Copying the class from highpass filter and applying the require changes

[yger]

This is an interesting feature, but be careful about the "magic" parameter margin_ms, that should be increased for small low pass frequencies. Indeed, this is the margin that will be consider to avoid border effect while filtering a chunk. While this parameter of 5ms is rather safe for high frequencies typically used in spike sorting, it might be too small for larger frequencies if you can for tens of Herz. (and then, the advanges of chunk based filtering might be lost also, because margin will be as long if not longer than chunk themselves). In fact, I think it would be good if margin_ms could be set automatically, to avoid any misusage by users. Why not setting it automatically to the lowest bounds of the frequencies considered ?

[jesusdpa1]

Thanks @yger! I think I understand but just to make sure you mean setting margin_ms to

fs = recording.get_sampling_frequency()
margin_ms = int((1 / (2.5*freq_max)) * fs / 1000)